1. Field of the Invention
The present invention relates to improved nickel aluminide single crystal base alloy compositions having superior tensile strength and stress-rupture strength and capable of being wrought or cast into shape by single crystal casting technology at a high or standard solidification rate.
Single crystal nickel aluminide alloys of different compositions are well known as proposed substitutes for single crystal nickel chromium alloys, or stainless steels, in the event that chromium becomes unavailable.
Nickel aluminide can be cast as single crystal Ni.sub.3 Al, or can exist as polycrystalline nickel aluminide. The Ni.sub.3 Al phase is brittle and drops in strength above about 1400.degree. F. The ductility of Ni.sub.3 Al has been improved by the minor addition of boron. However, greater improvements in strength and ductibility at elevated temperatures, up to about 1600.degree. F., are necessary to permit the use of modified Ni.sub.3 Al alloys for higher temperature applications.
2. Description of the Prior Art
It has been proposed to alter the properties of nickel aluminide alloys by the addition thereto of various ingredients.
U.S. Pat. No. 4,677,035 discloses high strength nickel base single crystal alloy compositions having high stress-rupture strength at elevated temperatures, such as 1800.degree. F./20 ksi for 1000 hours. Such compositions contain relatively high amounts of chromium and cobalt, have unsatisfactory stress rupture strength at low temperatures and have unsatisfactory oxidation resistance and corrosion resistance.
U.S. Pat. No. 4,885,216 discloses improved nickel base alloy compositions having similar high temperature stress-rupture strength properties as the alloys of U.S. Pat. No. 4,677,035 but having improved oxidation resistance and corrosion resistance due to the incorporation of small amounts of hafnium and/or silicon and optional small amounts of yttrium, lanthanum and/or manganese. However the alloys of this Patent also have unsatisfactory stress-rupture strength at low temperatures
U.S. Pat. No. 4,612,164 discloses the inclusion of boron, hafnium and/or zirconium in nickel aluminide alloys to improve ductility and yield strength up to about 133 ksi at elevated temperatures up to about 850.degree. C. (1562.degree. F). The addition of titanium, molybdenum and/or tungsten is not suggested.
U.S. Pat. No. 4,711,761 issued on an application referred to in U.S. Pat. No. 4,612,165, and discloses Ni.sub.3 Al alloys to which manganese, niobium and titanium are added to improve fabricability. The nickel aluminide alloys are doped with boron and a substantial weight of iron, but the amount of titanium is only 0.5 weight percent. Such iron-containing compositions have limited tensile strength and temperature capabilities.
U.S. Pat. No. 4,478,791 discloses the addition of boron to nickel aluminide alloys to improve the strength and ductility thereof, and U.S. Pat. No. 4,613,489 discloses that the loss of ductility of such cast composition during annealing can be avoided by subjecting them to hot isostatic pressing. Compositions containing specific amounts of titanium, molybdenum and/or tungsten are not disclosed.
U.S. Pat. No. 3,933,483 discloses the addition of at least 10% by weight molybdenum and up to 2.5% by weight of silicon to nickel aluminides in order to increase the tensile strength at elevated temperatures and the toughness at room temperatures without impairing the oxidation-resistance thereof. The addition of tungsten and/or titanium is not disclosed, and silicon is a melting point depressant.
Related U.S. Pat. No. 3,904,403 further discloses the addition of titanium, chromium, zirconium, niobium, tantalum or tungsten to silicon-containing nickel aluminide alloys. No compositions containing molybdenum, tungsten and titanium are disclosed.
Other prior art patents of interest include U.S. Pat. No. 4,461,751 and 2,542,962.